Measurement data relating to arterial stiffness and central blood pressure play an ever greater in assessing cardiovascular risks. Established medical diagnostic systems (SphygmoCor, Complior, Arteriograph) use piezoelectric, tonometric, or oscillometric measuring methods for obtaining such data from the blood pressure wave occurring in the circulation. Measuring methods of this kind can depict the time behavior of the blood pressure in a peripheral artery with such detail that the blood pressure function (pressure over time) can be reliably separated into individual subcomponents, even if they partially overlap. Usually, the first subcomponent is interpreted as a direct wave and the second is interpreted as a wave that is reflected at the branch into the two large pelvic arteries. The magnitudes of the two components and their time difference are then diagnostically relevant. Based on the time difference and the (doubled) length of the aorta, it is possible to calculate the pulse wave velocity, which depends on the blood pressure and vascular condition.
Various publications have explored the correlations between blood pressure and certain features of the photoplethysmographically depicted pulses. These studies focused on either the time difference between the so-called R wave of an additionally detected EKG and the starting point of the pulse wave or shape features of the pulse wave alone, determined without an EKG.
A photoplethysmogram (PPG) is generally understood to be an optically obtained plethysmogram, i.e. the measurement of a volumetric measurement of an organ. With regard to the present invention, a photoplethysmogram is depicted in order to determine the volumetric change in blood vessels, which is dependent on the blood pressure wave occurring in the circulation. Photoplethysmographic values can, for example, be detected using pulse oximeters, which supply a volume-dependent measurement value based on changes in light absorption in peripheral tissue through which blood circulates.
DE 10 2008 002 747 A1, for example, has disclosed a pulse oximeter in the form of an ear sensor. The ear sensor is used to monitor at least one physiological measurand by means of a noninvasive measurement in the ear canal. To do so, the ear sensor has a plurality of optoelectronic components, which are arranged in a housing that can be inserted into the ear canal, with the plurality of optoelectronic components being distributed around the periphery of the housing.
US 2013/0253341 A1 describes a device and method for noninvasive continuous blood pressure determination. To accomplish this, the data processing in a conventional photoplethysmographic measuring system is enhanced in order to enable continuous noninvasive blood pressure determination. It is apparent, however, that the photoplethysmographic pulse wave is significantly smoother than the peripheral blood pressure wave that was obtained according to the above-mentioned methods. For this reason, much fewer details can be distinguished in the photoplethysmographically determined pulse wave.
Particularly with the previously known photoplethysmographically functioning methods, it separation into direct and reflected subcomponents is not possible. Blood pressure changes can only be determined based on changes in relatively extensively blurred shape features. But since these features are also dependent on other variable physiological influencing factors, it is necessary to carry out a regular calibration with a reference blood pressure measuring system.
US 2014/0012147 describes a device and method for continuous noninvasive blood pressure measurement, which should enable an automatic recalibration. In this case, reference is made to a duration ΔT between a first and second maximum in the signal curve, but this could not be brought into relation to the above-mentioned pulse wave transit time.
U.S. Pat. No. 6,616,613 B1 discloses a device and method for monitoring physiological signals such as the blood volume contour. To achieve this, a photoplethysmographic sensor is positioned on a user's body part. Based on the electrical signals of the sensor, physiological parameters are determined that are then processed. Non-pulsatile and slowly pulsing signals are filtered out from the blood volume contour. Characteristics of the user's aortal reflected wave contour are extracted from a volume contour, with the volume contour being selected from the blood volume contour and the filtered blood volume contour. The characteristics of the user's aortal reflected wave contour are determined in part from the fourth derivative of the volume contour. The physiological parameters are shown to the user.